MYO15A is one of the most common pathogenic genes for NSHL. Indeed, after GJB2 and MYO7A, MYO15A is the third most commonly identified gene associated with ARNSHL (https://hereditaryhearingloss.org). In this report, we found pathogenic compound heterozygous mutations in MYO15A, including a novel missense mutation c.6353T > C (p.Leu2118Pro) that has not been reported previously. PTA and ASSR of the proband showed profound deafness. In addition, the patient/s ABR and DPOAE were still negative under strong stimulation, suggesting that the patient was severely deaf. Moreover, Sanger sequencing confirmed that the new mutation p.L2118P co-segregated with the disease phenotypes of the family, and strong genetic evidence was obtained using sequence conservation analysis and prediction of the protein structure changes caused by mutations at those specific points.
Myosin XVA is an unconventional myosin encoded by the human MYO15A gene and consists of 3530 amino acids. It is expressed at the tips of hair cell stereocilia in the inner cochlear region [7–9]. Three evolutionally conserved regions (head, neck, and tail) are present in this protein. The head region includes the N-terminal domain and the motor domain. The motor region plays a key role in ATP activity and contains two binding sites for adenosine triphosphate and actin. The neck contains two IQ motifs associated with calmodulin light chain binding. The longest tail region includes two MyTH4 domains, two band F/ezrin/radixin/moesin (FERM) domains, a Src-homology-3 (SH3) domain, and the C-terminal isoform I and PDZ-binding ligand domain [10–13]. The novel missense mutation, L2118P, found in this study was in exon 29 of the first MyTh4 domain. In studies performed in different countries, several mutations, including c.V2114M, p.R2124Q, p.R2146Q, and p.A2153fs, have been reported in the first MyTh4 domain [14–17] and these mutations may lead to deafness and play an important role in the functions of myosin XVA.
Mutations that cause HL were first identified at the deafness, autosomal recessive 3 (DFNB3) site in the Bengkala kindred and two unrelated consanguineous Indian families [18]. Since then, many countries in South Asia, such as Pakistan, India, and Turkey, have reported various mutations [5, 19–23]. In East Asians, however, the first mutations in MYO15A were not reported until 2013 [16]. A study had shown that the p.The R2146Q mutation broke the surface links between MYTH4 and FERM and caused deafness. The p.The R2146Q protein contains a hydrophobic sac similar in structure to that of R1190 of MYO7A myth4-ferm reported previously. Other mutations reported in the MyTH4 region of myosin XVA have been shown to interfere with the interaction between myosin XVA and whirlin, thus inhibiting the formation of the complex needed for normal hearing. [15]. The MyTH4 region is closely associated with the microtubule binding and acting binding on the plasma membrane [24]. It has been shown that the MyTH4/FERM domains in myosin XVA are necessary for myosin XVA to locate to the tip of the stereocilia, which is the key to the formation of the transmembrane actin microfilament assembly complex [24–26]. Homozygous shaker-2 (sh2) mice show profound deafness similar to that observed in humans with DFNB3, but vestibular defects not present in human patients. The stereocilia of hair cells in sh2 mice are located correctly during development, but they are much shorter and lack the typical stepped structure compared with wild-type mice [27]. The other mutation, c.1185dupC, was first reported in 2012 [28] and is a frameshift by the introduction of an eventual stop codon in the MYO15A open reading framework (p.E396fsX431).
With the widespread application of next-generation sequencing technology, reports on MYO15A mutations are no longer limited to the Middle East and other countries, where inbreeding is common. Reports of MYO15A mutation sites are increasing worldwide, particularly for complex heterozygous mutation sites, resulting in continuous enrichment of the MYO15A gene mutation database.
In summary, a novel mutation of the MYO15A gene was identified in this study. The mutation was in the first MyTH4 domain of myosin XVA and affected its protein function. The results of this study indicated that second-generation target region sequencing was feasible for the identification of rare variants in HL. Our findings expanded our knowledge of pathogenic variants in the MYO15A gene in patients with ARNSHL and could have implications in genetic counseling for families with HL.